So-called “lost wax” or “investment” casting methods are particularly suitable for producing metal parts that are complex in shape. Thus, lost wax casting is used in particular for producing multi-vane nozzle guides for turbomachines.
In lost wax casting, the first step is to make a model out of a sacrificial material that may be eliminated at a melting temperature that is comparatively low, such as for example a wax or a resin, with the mold being made by being overmolded on the model. Once the mold has consolidated, the sacrificial material is discharged from the inside of the mold, which is referred to as a “shell” mold. Thereafter, molten metal is cast into the mold so as to fill the cavity left in the mold after the model has been discharged therefrom. Once the metal has cooled and has solidified completely, the mold may be opened or destroyed in order to recover a metal part having the shape of the model made of sacrificial material.
In order to be able to produce a plurality of parts simultaneously, it is possible to combine a plurality of models made of sacrificial material in a single cluster, with each sacrificial material model of being connected to at least one structure, generally a central shaft that is not made of sacrificial material and to a distribution ring that is made out of sacrificial material. Inside the mold, the ring forms casting channels for the molten metal, also referred to as the feed system.
In the present context, the term “metal” covers both pure metals and metal alloys.
Multi-vane nozzle guide sectors are commonly made by assembling together a plurality of single-vane models made of sacrificial material so as to build up a multi-vane model made of sacrificial material. Nevertheless, because of the shape that is desired for certain parts of the nozzle guide, it may be found difficult to keep the dimensions and the shape of the model made of sacrificial material after it leaves the injection mold, and this applies in particular for the platforms of the vanes of the nozzle guide whenever they present a size that is relatively large and a thickness that is relatively small, e.g. forming a relatively large overhang.
Specifically, at the outlet from the injection mold used for making the single-vane model, the single-vane models made of sacrificial material are at a temperature where irreversible and major deformation may occur to the shape of the single-vane model. This risk remains present until the single-vane model made of sacrificial material has stabilized at ambient temperature.
Furthermore, prior to assembly, each single-vane model is subjected to non-destructive inspection of the single-vane model in order to verify that the single-vane model complies with the manufacturing dimensions and tolerances. When a plurality of single-vane models are judged to comply with specifications, they may be assembled together with one another so as to build up a multi-vane model made of sacrificial material.
During these steps, the handling of the single-vane models and of the multi-vane models may also give rise to risks of the single-vane or multi-vane models being deformed, and in particular to risks of their platforms being deformed.
Such deformation may also occur during subsequent steps of handling multi-vane models made of sacrificial material, such as for example while forming the shell mold, e.g. while the multi-vane model made of sacrificial material is being dipped in a slurry.
Unfortunately, the dimensions and tolerances for multi-vane nozzle guides may not accommodate such unwanted deformation, so multi-vane models that are not in compliance may not be used for making a shell mold. Specifically, the use of non-compliant multi-vane models would lead to scrapping of the multi-vane nozzle guide sectors that are obtained as a result of casting metal into the shell mold that is obtained from such non-compliant multi-vane models, in particular when the platforms present unwanted deformation having a direct influence on the size of the fluid flow passage.
In the disclosure below, the terms “inner” and “outer” are defined relative to the central axis of the turbomachine in which the elements are to be assembled, with the term “inner” relating to an element that is closer to the central axis than an “outer” element, and the terms “upstream” and “downstream” are defined relative to the normal flow direction of the stream through the turbomachine.